I finally got around to blogging about this study published in PLoS One a few weeks ago, regarding geophagy in tropical species of bats. The study provides a nice overview of the literature and some of the potential reasons why they (and we) do it.

We all eat dirt, in a sense, through mineral supplements or through the minerals and inorganic nutrients contained in our food, but there is a long history of the consumption of clay by human beings, and some tribes in sub-Saharan Africa continue to visit these “clay licks”. Pregnant women in particular will frequent these licks. Scientists believe that the clay is soothing to morning sickness as well as a good source of calcium for the skeleton of the developing child.

So the fact that animals of all kinds eat dirt is well established, but it’s hard to pin down exactly why in specific situations. There are two main hypotheses that attempt to explain why, both of which were considered by the researchers involved in this study: Mineral licks provide specific nutrients to an inadequate diet and/or they provide a buffer against plant secondary metabolites (PSMs), compounds that serve an ecological function to the plant beyond sex and energy production. In this case, PSMs protect plants from ingestion by poisoning the consumer. (To truly cover how interesting PSMs are would consume another blog post at least, which I have scrawled in my notebook.)

In this particular study, the researchers captured two species of bats each with different feeding habits: Artibeus obscurus is an obligate frugivore while Carollia perspicillata is a generalist species, feeding on fruit, pollen and insects. Insects provide little nutrients compared to fruit, so the researchers expected to see a greater frequency of generalist visitors than strict frugivores. However, most of the bats captured at mineral licks in the tropics are frugivores and many of these are pregnant, lactating females.

So if a diet of fruit is generally more nutritious than a diet of insects, why are there more fruit-eating bats captured at the licks? Probably PSM, especially if you consider this: pregnant female frugivores are eating for themselves and their offspring, which means a greater intake of fruit and PSM within the fruit, leading to a greater need for the toxin buffer provided by mineral licks. In this case it seems that mineral licks are more frequently used for detoxification than nutrients.

It’s important to note that not all mineral or clay licks are the same obviously, so this isn’t a solution to the dueling hypotheses (which probably explain the behavior as a pair instead of separately), but it does provide a bit of insight as to how specific cases will differ depending on the chemical composition of the lick and the needs of the organism.